Solar Windjamming

In 1958, physicist Richard Garwin of IBM Watson Scientific Laboratory
wrote a short paper in the journal Jet Propulsion
explaining the possibility of "sailing" through space by
capturing the pressure of the sun's rays. All that is needed is a
thin sheet of reflective material. Solar photons bounce off and
transfer momentum to the sail, allowing the spacecraft to accelerate
without expending fuel. Garwin's was the first technical publication
to describe solar sailing in an English-language journal. But some
of his readers had probably learned about the concept seven years
earlier from an article by engineer Carl Wiley, published under a
pen name in the magazine Astounding Science Fiction. And
many who missed both Wiley and Garwin's papers were awakened to the
idea by Arthur C. Clarke's 1963 short story "The Wind from the
Sun," which describes a solar-sailing regatta.

In the decades since elapsed, this elegant means of space propulsion
has remained squarely in the realm of science fiction. Solar
sailcraft have not been built, largely because formidable technical
difficulties arise when one tries to design a probe that can unfurl
a gossamer sail that is many tens of meters—or perhaps even
kilometers—across, without using heavy booms or guy lines.

But a design for a huge yet easily deployed solar sail seems finally
to have appeared on the horizon. Curiously, the key innovation comes
not from some recent progress in aerospace engineering or thin-film
technology. Rather, the advance results from a new way to think
about solar sailing—while remembering some basic science about
the interaction of the Earth and Sun.

Geophysicist Robert Winglee of the University of Washington has
studied solar and terrestrial magnetospheres for the past 15 years.
Like many physicists who examine the interplay of natural magnetic
fields in space, Winglee was aware that solar-wind protons racing
outward from the sun at 500 or more kilometers per second transfer
some of their momentum to the Earth when they are deflected by our
planet's magnetosphere. About five years ago, he started to consider
the possibility of harnessing this effect for propulsion. He
imagined that a space sail could capture the momentum of solar-wind
particles rather than solar photons, as Wiley, Garwin, Clarke and
many others had always envisioned.

Winglee realized that a simple coil would not do the trick, because
its magnetic field, which diminishes with the cube of the distance,
would not extend far enough to catch much of the solar wind.
Reviewing the literature, he discovered that some visionaries had
considered building a "magsail" using an immense
superconducting coil, perhaps hundreds of kilometers in diameter.
This scheme seemed far too impractical to merit much attention. But
Winglee's background in geophysics led him to a key insight. Instead
of trying to construct a huge superconducting loop in space, he
envisioned a way to inflate the magnetic field from a compact coil
into a billowing "mini magnetosphere" by spewing out
ionized gas—that is, a plasma.

Following the well-known principles of electromagnetism, the
magnetic field lines from the coil would, in a sense, be frozen into
the conductive plasma. Then, as the plasma cloud expanded around the
spacecraft, so too would the magnetic field. This mechanism would
thus allow the craft to unfurl a huge magnetic sail—tens of
kilometers wide—with no rigging other than the field lines.

"It's a very simple concept," notes George Parks, a
space-plasma physicist who collaborates with Winglee at the
University of Washington. When Winglee first suggested this
strategy, Parks doubted that enough force could be attained. But
Winglee now has him firmly convinced. Dennis Gallagher, a
magnetospheric plasma physicist at NASA Marshall Space Flight
Center, is also enthusiastic: "The idea of using an artificial
magnetosphere for a spacecraft for propulsion—that's really
cool."

Others at the space agency feel similarly: The NASA Institute for
Advanced Concepts funded Winglee to mount a pilot study last year
and has recently awarded him a grant to continue the work for
another two years. Informal research groups at Marshall Space Flight
Center and at the NASA Jet Propulsion laboratory are now gearing up
to evaluate whether this novel method could prove as useful as early
projections promise.

According to Winglee's estimates, a 200-kilogram probe could deploy
a magnetic sail of perhaps 20 kilometers' breadth and attain a
velocity of nearly 100 kilometers per second using 50 kilograms of
gas and about 1,000 watts of power to keep the plasma envelope
filled. Making way at that clip, a craft could reach from Earth to
Saturn in less than six months. The Cassini probe now on route to
the ringed planet, by comparison, will take seven years.

A round trip would also be possible, perhaps with a larger robotic
probe that could collect samples. "The device, if it does
scale, would be terrific for return trips to Mars or return trips to
Jupiter," notes Winglee. His sail, just like the thin-film
varieties that have been discussed for decades, could be canted at
an angle to the solar wind, so as to slow the orbital speed of the
vessel and allow it to drop toward the Sun. In the language
frequently used by advocates of solar sailing, the craft can be
"tacked" to travel upwind.

Indeed, using sailing terminology to discuss this sort of propulsion
has proven irresistible. In some respects, such metaphors are even
more apt for Winglee's sail, which would truly catch the solar wind
(that is, the charged particles emanating from the Sun) rather than
rely on the momentum of reflected photons. But Winglee's concept may
also need to borrow some of the vocabulary of ballooning, because
unlike traditional solar sails, it requires an expendable gas
(helium is under consideration) to fill its magnetic envelope. And
like a weather balloon rising in the atmosphere, the magnetic bubble
of Winglee's sailcraft would grow in size as it moves away from the
Sun. This expansion would compensate for the ebbing of the solar
wind as the craft traveled farther out, thus providing a constant
propulsive force. In this respect, Winglee's version is superior to
all previous proposals for solar sails, which would become less
effective with increasing distance from the Sun.

Whether this promising design will ever be built will depend, in
part, on how successful Winglee is in crossing between disciplines,
from geophysics to aerospace engineering. Appropriately he has
impressed John Slough, a professor of astronautics and aeronautics
at the University of Washington, James Burch, of the Southwest
Research Institute's Instrumentation and Space Research Division,
and Anthony Goodson, an aerospace engineer at Boeing, to join his
research crew. One can't help but wish them smooth
sailing.—David Schneider